a) Field of the Invention
The invention relates to a method of producing open-celled metal structures using a lost positive pattern comprising an open-pored substrate of plastics material containing pores with an average width of from 1 to 3 mm, in which method, after a negative pattern forming a core containing voids has been made, the voids are filled with metal by casting and the negative pattern is finally removed.
b) Description of the Prior Art
A method of this kind, although for producing an implant to serve as a bone replacement, is known from DE-OS 3106917, which corresponds to U.S. Pat. No. 4,781,721. In this case the voids in a sponge of natural or synthetic material forming the positive pattern are filled with a ceramic encapsulating material, the material of the pattern is removed by applying heat and the voids in the negative pattern so obtained are filled with metal. Finally the ceramic core material is removed.
A problem which has arisen with this method is that the walls and the interlinking webs of a natural or synthetic sponge are so weak that the shaped metal body produced by the method is not sufficiently strong.
In DE-OS 32 24 265 (which corresponds to U.S. Pat. No. 4,600,546, the above method is refined by increasing the thickness of the walls and/or the interlinking webs of the pores in the shaped plastics body which is used in this case as the positive model with a deposit of wax, at ambient temperature, either by dipping the pattern in liquified wax or a wax-in-water emulsion or by spraying the pattern with liquified wax or a wax-in-water emulsion, after which water evaporates and the dry wax which is left is then protected with a coating of synthetic lacquer.
Within certain limits, this method allows an open-celled shaped metal body to be produced in which the webs of the cells are thicker than in the shaped body produced by the first method.
However, a disadvantage of this method is that the wax applied onto and into the shaped plastics body by dipping or spraying, combined with the lacquer sprayed onto it, behaves as a rigid structure in comparison with the shaped plastics body, which is elastic, with the result that, under pressure, the wax may separate again slightly from the walls and webs of the pores in the shaped body. Another major disadvantage of this method of application is that the polymeric surfaces of the walls and webs of the shaped plastics body take on a negative charge vis-a-vis the wax or the wax-in-water emulsion. Consequently, there is not a circularly uniform bond between the walls and webs of the shaped plastics body and the wax. This is particularly true of the walls and webs situated within the shaped plastics body in the depthwise direction, or in other words at a distance from its surface. Hence the rheological characteristics of the wax or the wax-in-water emulsion within the shaped body are not sufficiently good to allow the walls and webs within the positive pattern in the depthwise direction to be thickened to a satisfactory degree. As a result the shaped metal body which is produced by this method contains within it walls and webs which are in some cases too thin to withstand the stresses applied in the long term.
Hence against the background which has been outlined, the object of the present invention is to refine the method described above in such a way that the walls and webs of the positive pattern are reliably thickened everywhere, i.e. within the pattern in the depthwise direction as well.